Her Wily Duke by Arietta Richmond

A second son, aghast at watching his brother fritter away his inheritance, dishonouring the title, a young music teacher, accidentally drawn into the secrets and scandals of a Dukedom, a notorious highwayman, an impossible love, a miraculous resolution.

Adam Villier, Marquess of Whitewell, is far better suited to being the Duke than his older brother – but that isn’t an option, of course, for the only way that Adam could become Duke would be if his brother died – which Adam certainly would not wish for! But, as Adam watches his brother waste away what his father and grandfather built, he is desperate to find a solution, to save the estates, and the tenant farmers and villagers who depend on it. He is driven to ever more desperate measures, even whilst he finds himself captivated by his sister’s music teacher – but how could he even consider a relationship, when there is so much that he must do, before disaster strikes?
Miss Emme Mowatt enjoys teaching Lady Caroline Villiers, but feels almost threatened by the Duke – yet he is her employer, so she really cannot complain… When his younger brother, Lord Whitewell, begins to be kind to her, to show some interest, she is torn. Can he be serious? Surely not, for she is just a commoner. Nonetheless, she finds herself drawn to him, more and more, her heart hopelessly engaged.